The dinghy slowed to a stop at a long line of black bobbing baskets and David Lawlor reached out to inspect the first one.

Inside lay 60 oysters, all with their shells closed, shielding the life within. “They look great,” beamed Lawlor. So did their neighbours in the next basket and the ones after that, all down the line of 300 baskets, totalling 18,000 oysters.

They are, however, never to be eaten. Instead they are tasked with reproducing and restoring oyster reefs to Dublin Bay more than two centuries after they were wiped out. “We want them to live long and happy lives,” said Lawlor.

Baskets filled with oysters in Dún Laoghaire harbour in Dublin Bay. Photograph: Rory Carroll/The Guardian

This pioneering project in Dún Laoghaire harbour is betting that a species that thrived here for millennia – before the waters became an oyster graveyard – can do so again.

Similar restoration projects are unfolding elsewhere in a continent that once had sprawling reefs of the European flat oyster (*Ostrea edulis) *until overfishing, dredging and pollution wreaked obliteration.

Reefs create rich ecosystems, provide a habitat for almost 200 fish and crustacean species and play a vital role in stabilising shorelines, nutrient cycling and water filtration.

“These oysters are amazing climate heroes,” said Lawlor, co-founder of Green Ocean Foundation, a nonprofit that is driving efforts in Dublin. “They are natural filter feeders. Each oyster filters at a rate of 190 litres of seawater a day.”

By feeding on plankton and nitrates, the oysters clear algae and help sunlight to reach the seafloor, boosting sea grass – a carbon sink – which in turns helps other species and improves coastal biodiversity and marine habitat.

Ireland’s inhabitants cultivated oysters in the middle ages but in the 1800s industrialisation and overfishing killed off the Dublin Bay reefs – a phenomenon replicated from Scandinavia to the Mediterranean.

Inspired in part by New York’s Billion Oyster Project, Lawlor enlisted volunteers and business sponsors for pilot projects that moved oysters from Tralee Bay in County Kerry to sites in Malahide, Howth, Poolbeg and Dún Laoghaire, which ring Dublin bay, and in Greystones, in County Wicklow.

Lawlor inspects oysters laid in Dublin Bay by the Green Ocean Foundation. Photograph: Rory Carroll/The Guardian

“You’re building your understanding of why things work well or don’t work well. You want to make sure they survived, to see what the growth was like, and to see if they spawn,” said Lawlor.

The transplanted oysters fared especially well in Dún Laoghaire so it was chosen for the next phase of the project – last November volunteers placed 300 baskets with 18,000 adult oysters in a sheltered part of the harbour. It is hoped they will become broodstock – spawn baby oysters in summer that will settle around the harbour and, in time, create a reef.

Scientists from Dublin City University’s Water Institute analysed the water last year for baseline indicators and will monitor the oysters’ impact with sensors and chemical and biological assessment.

The baskets are connected along a 100-metre line and are flipped by hand every few weeks to let Arctic terns, gulls and other birds to peck away fouling that might otherwise curb the flow of water through the baskets.

In Northern Ireland, the charity Ulster Wildlife used a different technique recently to place 2,000 adult oysters and 30,000 juveniles, sourced from Scotland, on the Belfast Lough seabed.

Aoibheann Boyle and Andrew Collins inspect oysters at the Green Ocean Foundation’s project in Dún Laoghaire harbour. Photograph: Rory Carroll/The Guardian

The Luna Oyster Project, a collaboration between Norfolk Seaweed and Oyster Heaven, aims to restore 4 million oysters to the North Sea by using the first mass deployment of clay structures called mother reef bricks.

The Dublin initiative is far smaller but will hopefully grow, said Lawlor. “The temptation is to think massive but you need to take one step at a time. A lot of the challenge is bringing people with you,” he said, citing government departments, local councils, wildlife groups and harbour authorities.

Last weekend, accompanied by volunteers Andrew Collins and Aoibheann Boyle, he returned to Dún Laoghaire, a wealthy, liberal neighbourhood, and boarded a dinghy to flip the baskets.

Under a winter sun the trio recorded clips for the Green Ocean Foundation’s social media accounts and fielded supporters’ queries. One, sent in jest, proved unanswerable: “Can the oysters filter the smugness out of the people of Dún Laoghaire?”